The invention relates to methods for producing a conversion element for an optoelectronic component. The invention further relates to an optoelectronic component, in particular a light-emitting diode.
There is a need in the industry for optoelectronic components for a down-conversion material that, when combined with the blue light-emitting chip, generates a warm white spectrum with good color rendering. This generally implies that at least two phosphor compositions are required, for example a green garnet-type phosphor and a red nitride-type phosphor. Additionally, this down-conversion material must simultaneously tolerate both high blue light fluxes and high temperatures over long periods of time. The standard ways of making down-conversion materials all have drawbacks when a warm white, high CRI solution is needed. For example, the familiar phosphor and silicone down-converters, which are found in many products today, are not sufficient because the silicone materials degrade rapidly at high light flux and temperatures. Single crystal and ceramic down-conversion materials can tolerate high light flux and temperatures because they have relatively high thermal stability and thermal conductivities, but they are generally limited to one phosphor composition, which means that they cannot produce the required warm white spectrum. The phosphor-in-glass and phosphor-on-glass down-conversion approaches are suitable for accommodating multiple phosphor compositions but only if the processing temperature is kept below about 350° C. or the red nitride phosphors start to degrade. The phosphor-in-glass and phosphor-on-glass down-converters have better thermal stability than the phosphor and silicone down-converters but their thermal conductivities are inferior to the ceramic or single crystal alternatives. The downside of the phosphor-in-glass or phosphor-on-glass approach is that the most stable glasses require processing temperatures above 350° C. There is still a need for a material that can accommodate more than one phosphor composition, is stable at high temperatures and high light flux and can be processed safely at under 350° C.
On surface-emitting, or thin-film type, optoelectronic components, like LEDs, the top surface of the LED chips is where the major share of light is emitted. For such LEDs it is common to place the down-converting element as close to the top surface as possible. One of the main reasons for this is thermal management. Stokes heat generated in the down-conversion process can be effectively removed through the LED device. Most thin-film type LEDs require at least one wire bond near the top surface in order to supply electricity. These wire bonds protrude above the top of the chip. The distance above the chip surface depends on the design of a given product but is typically in the order of about 125 μm. These wire bonds are fragile and cannot be exposed so that they are typically encapsulated in a protective material. For high-luminescence applications it is an often used practice to glue a down-converter directly on the chip surface. Then, to reduce emission from the sides of the down-converter, the sides of the converters are covered by a highly reflective material. The most common reflective material is titanium dioxide powder dispersed in a silicone. Not only does the titania-in-silicone prevent side emission, it also provides protection for the bond wires. This places a limitation on the thickness of the down-converter. It should be at least as tall as the distance by which the bond wire protrudes above the chip so that the side casting process completely covers the wire bond but does not block the forward light path.
Therefore, for uses in an LED package, the down-converter needs to be about 125 μm thick or even thicker. This thickness limitation is not ideal from a thermal management point of view. It is better if a conversion element is located in a more concentrated area near the chip. In this way, the Stokes heat can be removed from the phosphor through the LED chip. Therefore, there is a need for a conversion element that has the conversion material concentrated in a small volume with a thickness of less than 150 μm. The remainder of the 150 μm thickness can come as an additional layer or additional layers that do not contain down-conversion material.